1. Field of the Invention
Our invention relates to the field of communications, and more particularly, to reduction or elimination of group delay distortion resulting from filtering an information carrying analog signal
2. Description of the Prior Art
When an analog signal is digitized, it is common to take samples at a rate somewhat greater than twice the bandwidth of the signal. Because the transmission bit rate is proportional to the sampling rate, a low sampling rate is usually selected. In order to make digitization more accurate, the analog signal is first filtered to remove all frequency components except those of the desired signal. However, when the analog signal is filtered it undergoes a time delay. Over a large portion of the middle of the pass band, this delay is essentially constant. Approaching the edges of the pass band, however, the delay begins to drop off rapidly and a frequency dependent variation in the delay is introduced.
FIG. 1 illustrates a representative delay curve for an analog Bessel low pass filter having a cutoff frequency of 3200 hz. From this curve it is apparent that the time delay is relatively constant between 0 and 2000 hz. At frequencies near the cutoff, however, the delay has begun to drop and is measurably less than it was below 2000 hz. The Bessel Filter has delay characteristics which are much better than those of other well known filters. All analog filters, however, exhibit this undesirable feature to some extent. This phenomena, wherein some portions of the frequency spectrum have greater time delay than others, is referred to as group delay distortion or differential time delay. It obviously constitutes a degrading of the original signal and is not normally desirable. Because nearly all analog signal processing includes filtering, group delay distortion is a pervasive problem, the undesirable effects of which are magnified whenever multiple filtering steps are required.
Digital communications technology is more readily capable of signal processing without introduction of differential group delay. If the signal to be processed is initially in analog form, however, the very act of digitization of the signal will result in unwanted distortions. Conversion of the digital signal back into its original form will introduce still further differential time delay. It is desirable to eliminate all such delays from analog signal processing. Our invention achieves this result by a novel process which utilizes two known concepts, i.e., (1) it is relatively simple to construct analog filters which have almost no differential group delay at frequencies which are not near the band edges of the filter, and (2) digital filters may be relatively easily implemented with no differential time delay.